Unconventional measurement transformers make use of optical sensors. Each optical sensor is a combined system comprising an optical probe (e.g. of the glass ring type) and an optoelectronic circuit card connected to the probe by optical fibers. The optical probe is for placing in the vicinity of the electrical magnitude to be measured (i.e. close to the high voltage primary conductor). It includes a special sensitive medium which presents a slight modification in structure on receiving a magnetic or electric field. A light beam generated by the optoelectronic circuit card is injected into the probe and the reflected light beam serves to measure the electrical magnitude in application of well-known principles of the Faraday effect type for measuring the current carried by an electricity line or of the Pockels effect for measuring the voltage of an electricity line. The optoelectronic circuit card has an output on which it delivers digital data representative of the measured electrical magnitude, after treating the optical signal picked up by the probe. The term "optical current sensor" is used below to designate an optical sensor of the Faraday effect type for measuring the current carried by an electricity line, and the term "optical voltage sensor" is used to designate a Pockels effect type optical sensor for measuring the voltage of an electricity line.
Thus, optical current sensors are known for measuring electric currents with a measurement dynamic range adapted to a protection function (model "BK7" from "SCHOTT") or with a much smaller dynamic range but with measurement accuracy adapted to a measurement function (model "SF57" from "SCHOTT"). Also, patent document FR 93/01991 discloses an optical current sensor of large dynamic range adapted to a protection function and having a glass ring common to two independent outputs (i.e. two optoelectronic circuit cards).
With very high voltage, the way in which a series of advantages (not recalled herein) is obtained makes it seem likely that such unconventional transformers having optical sensors are going to replace so-called "conventional" measurement transformers based on toruses of ferromagnetic material with copper wires wound thereon (serving as a secondary winding) needing to be directly connected to low voltage protection and/or measurement equipment.
Nevertheless, the use of such unconventional measurement transformers raises problems of reliability and availability because of the presence of the optoelectronic circuits which are generally complex and less reliable than mere copper wires.
In particular, the optoelectronic circuit cards of the optical sensors are generally guaranteed for a mean time between failures of five years which is to be compared with thirty years as stated in the specifications for conventional measurement transformers.
To mitigate that drawback, it is therefore necessary to provide a certain amount of optical sensor redundancy, i.e. to provide within an unconventional measurement transformer a plurality of optical sensors for measuring the same electrical magnitude. Redundancy techniques with majority voting are well known. Thus, it is possible to provide an unconventional measurement transformer with three optical sensors for each phase of the electricity line and for each electrical magnitude (current or voltage) to be measured, the sensors having respective outputs connected to data processing means organized to output the data as produced by one of the output paths from the three sensors in application of the majority vote principle. Nevertheless, that configuration turns out to be very expensive in practice. For each electrical magnitude to be measured and for each phase it requires three optical sensors for the protection function plus three optical functions for the measurement function giving a total of six optical sensors.